Inert fillers, i.e. fillers which do not react with the base polymer, are commonly added to thermoplastic polymers, their main functions being to reduce the average cost of the product, the cooling times of the shaped piece and to provide the filled polymer with different technical features with respect to the initial polymer. Examples of fillers are calcium carbonate, silica, barium sulphate, talcum powder, wood powder or fibers, cellulose fibers and glass fibres. The amount of added filler generally is from 15 to 80% by weight of the polymeric base and preferably it is within the range of 30 to 70% by weight
Polymeric materials of interest are thermoplastic resins and particularly PP, PE and polyolefins in general, PVC ABS, etc; the material is a homo or copolymer and is a monomaterial or a mixture of compatible materials, e.g. PP/PE. In other words, mixtures of non-compatible materials are not within the scope of the present invention and with the wording thermoplastic material a monomaterial or a mixture of compatible material is meant.
Preferred thermoplastic material are industrial waste materials, such as production scrap, e.g. BO PP and PP/PE sheets scrap resulting from production of diapers and bi-oriented film, scraps resulting from production of non-woven fabrics and non-woven products, polymer powders deriving from polymerization processes and polymers productions.
According to the prior art, fillers are added to thermoplastic polymers after the polymeric material has been brought to a molten state. For example granules of polymeric material, or polymer material in a densified condition, are fed to an extruder and the filler is fed at one or more points of the cylinder of the extrusion screw after melting and plasticizing the polymer material; the filled material is granulated again when leaving the screw cylinder. The main disadvantage of this process is the cost of a process starting with material in the form of granules or pellets to again obtain granules or pellets of the filled material. This disadvantage is particularly felt when the polymeric material is a recycled material.
To obviate this problem it was proposed to fill the polymer granules or densified material during their processing step, for example during the extrusion or injection moulding of the shaped product; however, this solution proved to be applicable only in part, because the final user of the material prefers to use a compound already having the necessary amount of filler for obvious reasons of simplicity, lower costs of the process and cheaper and easier to use moulding and extrusion machinery.
U.S. Pat. No. 4,225,640 (Erb) discloses a method of reclaiming municipal waste by blending fragments of different and incompatible plastic materials and of dried leaves and by extruding the mix. Heat is applied in the extruder to fuse the thermoplastic materials and incorporate therein the leaf fragments. In one embodiment the thermoplastic materials are melted and fused before mixing them with the leaves fragments. The product leaving the extruder is a consolidated mass of leaves bonded by the plastics, where the leaf fragments serve as a bridge to integrate different thermoplastic materials that would not otherwise combine together.
DE-A-19718138 (Ermafa) relates to a recycling machine that fragments and agglomerates thermoplastic materials until C preset amount of material is treated in the machine; the material is then fused and “shock water” is added to solidify it within the machine. The solid and agglomerated material is then dried, fragmented and extracted from the machine. The document is silent about fillers.
U.S. Pat. No. 5,635,125 (Ternes) discloses a shingle obtained by grinding fiber-reinforced PVC material (namely garden hoses), mixing the ground hose with sawdust and extruding the mix. Again, in this document the filler is incorporated in the molten plastic material during extrusion.
U.S. Pat. No. 3,552,722 (Sutter) deals with the problem of uniformly mixing powdered plastic and fragments of plastic material before feeding the mix to an extruder or kneading and heating device. The disclosed apparatus does not carry out any shredding or softening of the material and no fillers are mentioned.
EP-A-0911131 (Gamma Meccanica) discloses an apparatus for fragmenting, shredding and densifying a plastic material. The apparatus comprises a container with a rotator that shreds the plastic material and pushes it into a first screw that is tangential to the container and that carries the material to an extruder screw. Feeding means are provided for feeding dyes or granulated plastic to the tangential screw. The cited arrangement should ensure a more uniform feeding of shredded plastic to the extruder.